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Chapter 1
Red Dragon Fruit: Using Red Pigment Extracts from Pitayas as Natural Indicators
Teo Tang Wee
Interesting Facts
1.Dragon fruit is native to Mexico, Central America, and South America.
2.This fruit is now commonly cultivated in East Asia, and Southeast Asia including Malaysia, Vietnam, Taiwan, the Philippines, and Thailand.
3.The plant can now be found in Japan, Hawaii, Israel, northern Australia, and southern China.
The Fruit of Dragons?
Pitaya is the fruit that grows on a red skinned climbing cactus known as
hylocereus (
Figure 1). This plant produces large white night blooming flowers known as “moonflower” or “Queen of the Night” and has a light melon taste. Pitaya is also commonly known as
(dragon pearl fruit) or
(fire dragon fruit) in Chinese,
thanh long in Vietnamese,
buah naga in Malay, and
kaeo mangkon in Thai. It has a beautiful bright red, leathery, and slightly leafy skin. The flesh of the fruit is translucent with tiny black seeds.
Figure 1. Dragon fruits sold at a fruit stall.
An excerpt from an online forum
I am very worried after seeing red urine and bowel movements this morning. It happened after I ate two pieces of red flesh dragon fruit last night. Should I go see the doctor? Does anyone have experience with this? Please share.
Relevant to Our Everyday Lives
There are generally three kinds of pitayas — red skin with white flesh, red skin with red flesh (Figure 2), and yellow skin with white flesh. All three types, in particular, the red skin dragon fruits are imported into Singapore and can be found in the local wet markets, supermarkets, and fruit stalls. It has gained increasing popularity as it is known to contain antioxidants. As the quality of education improves and people become more scientifically literate and health conscious, the choice of food beneficial for health becomes more popularly consumed.
Figure 2. The bright red colours of red dragon fruit flesh.
Figure 3. Hylocereus undatus.
Most fruits in Singapore are not grown locally but imported from other countries (Figure 3 shows a dragon fruit plant). Established in April 1, 2000, the Agri-Food and Veterinary Authority of Singapore (AVA) is the national authority on food safety for both primary and processed food. Among its many responsibilities, it overlooks the export and import of meat, meat products, fruits, and vegetables. AVA conducts routine laboratory testing of food samples, for example, to check for excessive pesticide residues. A fine and suspension will be imposed on the importer if the consignment fails to meet AVA’s regulated guidelines.
The red colouring in red beetroot is a commercial source of betacyanins — a type of water-soluble pigment. However, red beetroot contains geosmin and pyrazines that are responsible for the unpleasant peatiness of this crop and the high concentration of nitrates which can form carcinogenic nitrosamines (Phebe et al., 2009). As such, the red pigment in red dragon fruit may possibly be extracted and used as red colouring instead as it does not give the same negative sensorial feel.
Can the Pigment in Red Dragon Fruit Flesh be used as a Natural Indicator?
Chemical composition
The colour of dragon fruit red flesh is attributed to betacyanins, which is a class of water-soluble pigments (Wybraniec et al., 2007). Betacyanins are also found in red beetroots. The red pigments in red cabbages, often used to illustrate natural indicators in school-based experiments, are however, anthocyanins.
Betacyanins and anthocyanins have different chemical structures although both are water-soluble. With this knowledge, how would you address the anonymous writer’s concerns (on page 9)?
Guiding question
Red cabbage pigments can be extracted as natural indicators to test for acidity and alkalinity differences. Can the pigment in dragon fruit red flesh serve the same function?
Extraction procedures
Peel a red flesh dragon fruit and cut up the flesh into small chunks. Put the cut up fruit into a beaker and add enough water to cover the flesh (Figure 4). Stir the mixture to extract as much red pigment as possible. When the solution is sufficiently red in colour, filter the colouring into a conical flask using a filter funnel and filter paper (Figure 5).
Figure 4. Cut up dragon fruit flesh mixed in water.
Figure 5. Filtering the mixture to extract the red pigment.
Figure 6. Colours of the red dragon fruit extract in pH 1, 2, 7, 10, and 13 (from left to right).
Testing pH changes
The extract was added dropwise to solutions with various pH (1, 2, 7, 10, 13) and obvious colour change was observed at pH 10 (Figure 6). This shows that the red pigment in red flesh dragon fruit can be used as a natural pH indicator at high pH. What can you deduce about the pH of urine and bowel movements of the anonymous writer?
Student Inquiry
Suggested questions for student research
• What can be some possible commercial uses of the extracted red pigments from dragon fruit?
• How can the red pigment be extracted and packaged for commercial sale?
• What are some other fruits or vegetables that allow for colour pigments to be extracted and used as natural food indicators?
• What are the advantages and disadvantages of using natural indicators as compared to synthetic indicators?
• How are betacyanins and anthocyanins similar and different e.g., in terms of chemical structures?
• Why is the red colour pigment in red dragon fruit water-soluble?
• What happens to the chemical structure when it changes colour under different pH conditions?
• How would the titration results using the red pigment from dragon fruit flesh differ from synthetic pH indicators such as phenolphthalein and methyl orange?
Author’s note
• This activity is considered low-cost as two dragon fruit would yield sufficient red colouring extract for a class of 30 students. As students learn science, they will also learn not to waste food and resources. Cultivating good habits...
